A visual approach slope indicator (VASI) or a precision approach path indicator (PAPI) relies on the color variation of a multi-beam light source to guide a vehicle such as an aircraft to the correct glide slope. Incandescent, fluorescent or arc lamp based light sources have been employed in the previous disclosed VASI or PAPI apparatus. Some examples can be found in U.S. Pat. Nos. 4,034,480, 4,064,424, 4,183,078, 4,430,695, and 4,532,512.
In U.S. Pat. No. 4,034,480 to Mehrtens, entitled “VISUAL DESCENT SIGHTING DEVICE FOR AIRCRAFT”, a self-contained sighting device for use in aircraft that will give visual descent guidance to the pilot while making his final approach to a runway for a landing is described. The device comprises frames or windows that sight along an imaginary approach path which automatically directs the pilot's line of sight to parallel the desired approach path thereby aiding the pilot in his approach to the intended landing spot.
U.S. Pat. No. 4,064,424 to Hergenrother, describes a visual glide path beacon system. The beacons are placed in pairs at opposite ends of a runway so that it becomes possible to dispense with the conventional runway lights presently used to delineate the runway. In order to fill in the dark zone which surrounds the convergence produced when a lens is used to intensify the brightness of the colored zones, the invention uses an apertured mirror positioned opposite the lens. This permits the pilot to see the beacons through all parts of a traffic pattern as well as in the intensified colored zone guidance region. Instead of using a single light source projecting through a color filter to form the color zones, this invention uses two light sources such as incandescent lamps which project the color zones in slightly different directions which differ, by say, an angle of one degree. The two lamps are flashed on and off alternately producing a “scanning” action of the colored zone pattern.
U.S. Pat. No. 4,183,078 to Kidd, describes an approach slope indicator unit for use at the edge of airfield runways which produces a split beam of light, the interface of which may be adjusted to the required angle of flight path to guide incoming aircraft, the indicator unit comprising a support assembly having mounted therein at least two projection tubes, each projection tube housing a removable pre-aligned projector having a light source, a red-passing filter and a divergent lens disposed to produce the split beam of light.
U.S. Pat. No. 4,430,695 to Payne et al. describes a visual landing aid for aligning an aircraft landing aid projector of the type which comprises a light source located at the focus of a parabolic reflector, a lens mounted coaxially with the parabolic reflector and a filter disposed intermediate the reflector and lens close to the focal point of the lens. The apparatus comprises a base having a surface portion for supporting a projector, a lens for focusing light from the projector onto an image receiving means and means allowing observation of the image receiving means. The filter of a projector is correctly positioned when it appears in focus on the image receiving means.
U.S. Pat. No. 4,532,512 to Tanner describes an omnidirectional approach slope indicator unit for heliports. The unit may include two oppositely directed light beam generators secured to a rotatable shaft, the light beam generators generating light beams of differing colors to alert an approaching aircraft of his position relative to the glide slope indicated by the light beams.
The lamp based VASI or PAPI apparatus including the ones listed above suffer from a short life span of less than 2,000 hours (<2,000 hours). To solve this problem, laser based VASI or PAPI apparatus have been disclosed in U.S. Pat. No. 5,287,104 to Shemwell, which describes method and apparatus for aiding a landing aircraft. Three differently-colored beams of laser light are produced and transmitted, one of the beams of laser light being transmitted in a plane containing the direction from which the aircraft is approaching. Another of the beams of light is transmitted on one side of the plane, and the third of the beams of light is transmitted on the other side of the plane. The pilot of the aircraft can determine whether the aircraft is on the plane or to the one side or the other by the color of the light the pilot receives. If desired, at least one of the colored laser beams that is transmitted toward one side of the plane can be broken into a plurality of adjacent fan-shaped beams, the light in at least one of the fan-shaped beams being interrupted intermittently. However, the laser based VASI or PAPI apparatus typically suffer from high cost and low luminous power, or in other words, low luminous flux.
Therefore a need exists for an improved VASI or PAPI apparatus which offers a combination of long life time, low cost, and high luminous power.